In recent years, optical devices using infrared light have been actively developed. In particular, infrared light in the range of 8 to 14 μm have been attracting attention. Accordingly, various optical elements suitable for infrared light in this range have been proposed. As an example of such an optical element, an optical element having an infrared anti-reflective function is known which includes a substrate (base member) and an infrared anti-reflective film provided on the substrate. The substrate is formed of an infrared transmitting material such as ZnSe, ZnS, GaAs, or Ge. The infrared anti-reflective film includes a low-refractive-index layer made of a fluoride such as PbF2, BaF2, LaF3, CeF3, or ThF4, and a high-refractive-index layer made of, for example, ZnSe, ZnS, GaAs, or Ge.
In an optical element for use in a high-power laser, such as a CO2 laser, the influence of heat generated when the laser light is absorbed cannot be ignored. Therefore, it is necessary to reduce the laser-light absorption of the infrared anti-reflective film. Among the materials for forming the low-refractive-index layer in the infrared anti-reflective film, ThF4 is known as a material having such a characteristic. However, since ThF4 is a radioactive material, there are problems that ThF4 is harmful to humans and handling thereof is restricted.
To solve such a problem, Patent Literature 1 proposes an optical element including a low-refractive-index layer made of BaF2 and a high-refractive-index layer made of ZnSe.